Crystalline dipolymers of tetrafluoroethylene (TFE) and ethylene (E) can be produced over the entire composition range from polyethylene to polytetrafluoroethylene, creating polymers which melt between about 110.degree. C. and 327.degree. C. At about 50/50 molar ratio, where TFE/E copolymers are typically produced, a local maximum in the melting point curve is encountered. At this 50/50 ratio, a number of other properties also reach a local maximum (or minimum), such as modulus and crystallinity. This phenomenon was recognized by Carlson in U.S. Pat. No. 3,624,250. He defined a range of 60/40 to 40/60 which encompasses most of this local maximum. At the maximum, melting points of 270.degree.-285.degree. C. can be achieved.
Dipolymers of ethylene and tetrafluoroethylene have poor resistance to stress cracking, particularly at elevated temperature. Incorporation of a termonomer (Carlson, U.S. Pat. No. 3,624,250) was found to improve stress crack resistance and provide polymers which have found wide application in wire and cable coatings, films, and injection moldings. Many termonomers have been found to provide the desired cracking improvement in TFE/E polymers, including most common fluorocarbon and hydrocarbon vinyl compounds that introduce a bulky side group into the polymer. However, a limited number of such termonomers have been used in the commercial manufacture of TFE/E resins, including perfluorobutyl ethylene (PFBE), hexafluoropropylene (HFP), perfluoro(propyl vinyl ether) (PPVE), hexafluoroisobutylene (HFIB) and CH.sub.2 =CF(CF.sub.2).sub.3 H. Known TFE/E copolymers that are terpolymers are described, for example, in U.S. Pat. Nos. 3,960,825; 4,123,602; 4,513,129 and 4,677,175; and in Published Japanese Patent Application (Kokai) 07(1995)-041522.
Sulzbach in U.S. Pat. No. 4,381,387 discloses a quaterpolymer having a melting point of 245.degree.-280.degree. C. and consisting essentially of 55-30 mol % TFE, 60-40 mol % ethylene, 10-1.5 mol % HFP, and 2.5-0.05 mol % of a bulky vinyl monomer selected from one of a number of classes, including a class of perfluoro(alkyl vinyl ethers) (PAVE). This patent is said to provide TFE/E copolymers which provide a satisfactory compromise between thermal and chemical stability on the one hand and the tensile and elongation behavior on the other hand, with the use of a smaller quantity of a bulky vinyl compound. However, melting points achieved in the '387 patent are not low despite substantial incorporation of HFP. Sulzbach's Example 6 quaterpolymer has TFE/E/HFP/PPVE molar composition of 47.0/44.3/8.4/0.4 and a melting point of 247.degree. C. The Example 14 quaterpolymer uses perfluorohexyl ethylene (PFHE) instead of PPVE and has TFE/E/HFP/PFHE molar composition of 45.4146.5/3.7/0.2 and a melting point of 272.degree. C. Furthermore, it is well-known that the reactivity of HFP is low, so the use of a substantial quantity of HFP adversely affects polymerization rate. Sulzbach & Hartwimmer in U.S. Pat. No. 4,338,237 disclose a polymerization process for preparing TFE/E copolymers, including the quaterpolymers disclosed in the '387 patent.
In EXAMPLE I of U.S. Pat. No. 3,624,250, Carlson discloses a TFE/E/PPVE copolymer having molar composition of 48.8/48.8/2.4, respectively, and a melting point of 255.degree. C., and in EXAMPLE III the TFE/E/PEVE counterpart having a melting point of 262.degree. C., PEVE being perfluoro(ethyl vinyl ether).
TFE/E copolymers that have low stiffness and have good flex life at elevated temperature are needed for such applications as flexible, thin-wall appliance wiring and in coatings on large-diameter cables. Particularly desired are TFE/E copolymers having sufficiently low melting temperatures that the copolymers can be melt processed together with thermally less stable polymers such as fluorine-free polymers, i.e., co-processed such as by co-extrusion. Such composites are currently sought, for example, for fuel hose applications which require a combination of fuel resistance, ruggedness, and flexibility.